1. Field of the Invention
The present invention relates to a numerical control type oscillator whose oscillation frequency is controlled depending on set data.
2. Description of the Prior Art
A numerical control type oscillator (NCO) is an oscillator whose oscillation frequency is controlled depending on set data. Such a numerical control type oscillator is used to provide a signal synchronized with a carrier wave of a signal sent from a communication satellite, for example.
Clearly, in FIG. 1, an electric wave from the communication satellite is received at an antenna 41. The carrier frequency of the signal sent from the communication satellite may by varied due to the Doppler effect. The reception signal is converted into a predetermined frequency at a converter 42 which is provided near the antenna 41. The signal from the converter 42 is supplied to a down-converter 43. The signal is converted into an intermediate frequency signal with a predetermined frequency of several MHz at the down-converter 43.
The output of the down-converter 43 is supplied to a phase comparator 44. The output of a numerical control type oscillator 45 is also fed to the phase comparator 44. A phase comparison of the carrier wave from the down-converter 43 and the output of the numerical control type oscillator 45 is made at the phase comparator 44. The phase comparison output is given to an A/D converter 47 through a low-pass filter 46. The output of the A/D converter 47 is supplied to the numerical control type oscillator 45. The oscillation frequency of the numerical control type oscillator 45 is controlled depending upon the output data of the A/D converter 47.
As a result, a signal synchronized with the carrier frequency can be provided from the numerical control type oscillator 45.
FIG. 2 shows a structure of a conventional numerical control type oscillator used for such a PLL circuit.
In FIG. 2, reference numeral 51 is a controller for setting a frequency. A frequency setting value N depending on the output data of the A/D converter 47 is given to the controller 51. The frequency setting value N is fed to a frequency setting register 52.
The output of the frequency register 52 is supplied to an adder 53. The output of the adder 53 is given to an address register 54. The output of the address register 54 is supplied back to the adder 53.
A clock of a frequency F.sub.c is given to the address register 54 from a clock generator 55. The output of the frequency setting register 52 is sequentially accumulated at the adder 53 and the address register 54.
The output of the address register 54 is supplied to an address of a waveform generating ROM 56. Waveform data of one period is stored for an address of 2.sup.i, for example, at the waveform generating ROM 56.
The waveform data is produced from the waveform generating ROM 56 according to an address from the address register 54. The output of the waveform generating ROM 56 is supplied to a D/A converter 57. At the D/A converter 57, the waveform data from the waveform shaping ROM 56 is converted into an analog waveform. The analog waveform is taken out of an output terminal 58.
When the frequency setting value is N, an address A.sub.k generated from the address register 54 is advanced every frequency setting value N with the clock F.sub.c by the adder 53 and the address register 54. With the address A.sub.k advanced to 2.sup.i, a signal of one period is generated. For this reason, the time for the address A.sub.k to be advanced to 2.sup.i corresponds to the period of an oscillation output produced from the output terminal 58. The period T.sub.NCO is represented by ##EQU1##
As a result, an oscillation frequency F.sub.NCO is given by: F.sub.NCO =F.sub.c (N/2.sup.i). Consequently, frequency accuracy .DELTA.F.sub.NCO of the conventional numerical control type oscillator is expressed by EQU .DELTA.F.sub.NCO =F.sub.C .times.(1/2.sup.i).
In this way, in the conventional numerical control type oscillator, the accuracy of the oscillation frequency is represented by EQU .DELTA.F.sub.NCO =F.sub.C .times.(1/2.sup.i).
Therefore, to improve the oscillation frequency accuracy, it is needed to elevate the clock oscillation frequency F.sub.C or to increase an address bit number i.
However, with the clock frequency F.sub.C increased, it is necessary to accumulate the setting value N at high speed and to shorten the delay time at the adder 53. However, there is a limit to the reduction of the delay time of the adder 53.
Also, with the address bit number i increased, the circuit size increases correspondingly.